A commensal radar system is a radar system that utilises the emissions of other services without impacting at all on those emissions. A commensal radar system makes use of existing transmitters of opportunity to detect and range target objects, where there may be a single transmitter of opportunity and a plurality of separate commensal receivers, or vice versa. The transmitters of opportunity illuminate a scene with electromagnetic transmissions which may be constrained to a particular band, in accordance with the function of each transmitter. The separate receivers may then be configured to receive emissions from these transmitters which are reflected off target objects situated within the illuminated scene.
The term “commensal” is used as these radar systems typically operate alongside existing electromagnetic transmitters (transmitters of opportunity), utilising their transmissions without adversely affecting the electromagnetic transmitters or the intended recipients of these transmissions.
Target objects which may be detected by commensal radar systems can include cars, aircraft, ships, animals or humans. Targets are detected by cross-correlating a direct path reference signal (which is obtained directly from the transmitter of opportunity) with a surveillance channel signal. Peaks in the output of the cross-correlated signal correspond to targets. The delay at which a peak occurs corresponds to the bistatic range of the target, where the bistatic range is the range from the transmitter to the target to the receiver. A commensal radar system may also measure a Doppler-shift of the reflected transmissions so as to allow for moving targets to be differentiated from stationary targets.
Transmitters of opportunity can take the form of any electromagnetic transmitter. Some examples of transmitters of opportunity include GSM (Global System for Mobile communication) base station transmitters, Wi-Fi access points, broadcast transmitters (e.g. television or radio broadcast transmitters), or even space-born transmitters such as Global Navigation Satellite System (GNSS) transmissions. Favoured transmitters of opportunity are typically those with a relatively high output power and a reasonably high bandwidth. Transmitters meeting such requirements include digital audio broadcast (DAB) transmitters, analogue and digital TV broadcast transmitters and, to a lesser extent, FM radio broadcast transmitters.
The transmit power of the transmitters of opportunity influences the maximum detection range of the transmitters of opportunity, with a higher transmit power meaning that targets can be detected from a greater distance. The bandwidth of the electromagnetic transmissions being broadcast is inversely proportional to the resolution of the range measurements of the system. Electromagnetic transmissions of a higher bandwidth accordingly provide for improved differentiability of closely spaced targets.
Transmitters of opportunity are, however, not commissioned and/or installed with commensal radar functionality in mind. It is therefore a further requirement for favourability that the transmitters of opportunity be widely deployed so as to enhance the detection coverage of commensal radars.
Some of the challenges that exist for commensal radar systems concern poor signal to noise ratios, which may stem from difficulties in suppressing the direct path signal as well as relatively weak target returns. As the transmitters of opportunity are often continuous wave transmitters, the receivers should be able to operate concurrent to the transmitter, which means that they must be able to sufficiently suppress the electromagnetic transmission which propagates directly from the transmitter to the receiver. This is referred to as direct signal suppression, existing methods of which may include steering nulls of receiver antennas in the direction of transmitters of opportunity, making use of topography (i.e. using mountains to shield the receivers from the transmitters) or using software algorithms to suppress direct signals.
From a practical standpoint, null steering can be quite difficult as, in addition to the direct path electromagnetic transmissions, there can be strong electromagnetic reflections from stationary objects which may not fall in the antenna nulls. Topographical shielding is troublesome as the direct signal is still needed for target detection and software algorithms can be computationally intensive which may result in a higher system cost.
Some methods which have been proposed to overcome some of the difficulties involved in topographical shielding include receiving the reference signal at a separate receiver site and then communicating the reference signal and the surveillance signal to a central processor for cross-correlation. However such methods require communication links and accurate time stamping, which can be difficult to implement, especially in remote or rural areas.